Since a large number of absorption spectra unique to substances exist in the terahertz frequency range, measuring these absorption spectra allows a substance to be identified in a nondestructive and noncontact manner, then spectroscopic analysis or imaging using the characteristic of the terahertz frequency band has attracted attention and many researches have been conducted recently. However, a large-sized measurement system to be placed on a desk is conventionally required.
In conventional oscillators in a microwave band, a varactor diode can be regarded as a lumped constant circuit comprising a simple resistor R, an inductance L and a capacitor C and be thereby designed. In the terahertz frequency range, however, the wavelength is shorter than that in the microwave band and the size of the varactor diode is not negligible with respect to the wavelength. Therefore, an impedance of the varactor diode cannot be represented only by simple RLC and the varactor diode has a distributed constant characteristic where the impedance varies in a complex manner depending on the frequency.
In a conventional terahertz oscillator having a resonant tunneling diode and a slot antenna as described in Japanese Published Unexamined Patent Application No. 2013-171966 A (Patent Document 1), since the frequency is determined due to the area of the resonant tunneling diode and the size of the slot antenna, the frequency is thus not variable.
FIG. 1 illustrates a conventional terahertz oscillator. A layer of a lower electrode 4 is provided on a substrate 3 and a slot antenna 2 of a rectangular recess is disposed substantially in the central portion of the lower electrode 4. A layer of an upper electrode 5 is provided on the substrate 3, and a resonant tunneling diode 1 having a I-V characteristic as shown in FIG. 2 is disposed at a tip portion of the upper electrode 5 via a metal insulator metal (MIM) capacitor 6. When a bias is applied to the resonant tunneling diode 1, electrons tunneling via a quantum level in a well and thereby a current flows therein. When the bias is further applied, the electrons cannot tunnel any more when the quantum level in the well is lower than the bottom of a conduction band of an emitter and thus the current decreases, thereby resulting in the I-V characteristic as shown in FIG. 2. It is possible to amplify and oscillate the electromagnetic wave by using a differential negative resistance characteristic “−GRTD” with a decreasing current. The resonant tunneling diode 1 has a parasitic capacitance CRTD in parallel to the differential negative resistance −GRTD. The slot antenna 2 is represented by a resonant circuit of LC and a radiation loss Gant and thus an equivalent circuit of this oscillator is as shown in FIG. 3. The oscillation starting condition is at a time when a positive value GRTD of the differential negative resistance characteristic is equal to or more than the radiation loss Gant as expressed by the following Formula 1. The oscillation occurs at the frequency fOSC expressed by the following Formula 2.GRTD≥Gant  [Formula 1]
                              f          osc                =                  1                      2            ⁢            π            ⁢                                                            L                  ant                                ⁡                                  (                                                            C                      ant                                        +                                          C                      RTD                                                        )                                                                                        [                  Formula          ⁢                                          ⁢          2                ]            
As expressed by Formula 2, in the conventional terahertz oscillator, the value of the frequency fOSC is always fixed and thus the frequency fOSC cannot be variable. Therefore, development of a frequency-variable terahertz oscillator is desired.